No. 3.] FACTORS IN THE EVOLUTION OF MAMMALIA. 385. 



due primarily to the mingling of different hereditary tendencies, 

 especially when it is remembered that none of the ancestors of 

 these groups possessed any such teeth ? Or can it be reasonably 

 contended that such parallel variations are due to the direct 

 action of the climatic or other environment upon the germ- 

 plasm ? The tendency to the formation of prismatic molars 

 appears even in the early Tertiary times, as is seen in Paloplo- 

 therium and Hypisodus, but these were premature attempts, and 

 led to nothing. Cope has shown how clearly the skeletal pe- 

 culiarities of the feet of the Tylopoda may be deduced from the 

 mechanical effects of the cushion or pad upon which the foot 

 rests, and I may add that the steps of the enlargement of this 

 pad may be inferred, in the extinct forms, from the successive 

 changes in the phalanges. The similarities between the ele- 

 phants, on the one hand, and Uintatherium, on the other, are just 

 those which must accompany greatly increased height and bulk, 

 though the latter is the more important factor. 



Furthermore, the facts of pathology and surgery bring clearly 

 before us the way in which the skeleton is moulded by the 

 pressures and strains to which it is subjected. The tendinal 

 sulci are formed by the pressure of the tendons, and if these 

 tendons are dislocated, the old grooves are gradually filled up 

 and new ones formed. " After dislocations the old articular 

 cavities will be filled up and disappear, while at the new point 

 where the head of the bone is actually placed, a fresh articula- 

 tion is formed, to which nothing will be wanting in the course 

 of a few months, neither articular cartilages, synovial fluid, nor 

 the ligaments which retain the bone in place " (Marey, No. 36, 

 pp. 88, 89). The arrangement of the trabecular in the interior 

 of the bones is in the direction of the line of the greatest pressure 

 and strain, giving great strength with a minimum of material. 

 When, by a fracture and reunion, the relation of the bone to the 

 line of strain is changed, the direction of the trabecular will be 

 correspondingly altered. The papers of Arbuthnot Lane (Nos. 

 28 and 30) give an admirable account of the way in which long- 

 continued routine labor will modify the shapes and articulations 

 of the bones. It would, however, be a mistake to suppose that 

 the bone-structure is entirely due to influences exerted in the 

 lifetime of the individual, for the facts of development show that 

 these structures appear in the foetus before such influences are 

 exercised. 



